1. Field of the Invention
The present invention relates generally to a projector including primary color light sources such as Light Emitting Diodes (LEDs), and more particularly to a method and apparatus for calibrating a color temperature of a projector.
2. Description of the Related Art
In general, RGB light emitting diodes create a red light, a green light, and a blue light, respectively, i.e., primary colors, and a projector including RGB light emitting diodes creates a light of the desired color by mixing the primary color lights. The color temperature of the projector is measured for a light (generally, a white light) of a reference color output from the projector. The color temperature variation based on a target color temperature is generated for the white light output from the projector according to the brightness value and wavelength of the primary color light sources. The color temperature variation is influenced by a coupling error or a loss generated in the process of installing primary color light sources, lenses, and beam splitters in the projector.
FIGS. 1 and 2 are graphs illustrating a distribution of color temperatures. The distribution in FIG. 1 is caused by brightness and wavelength variations of primary color light sources. The distribution in FIG. 2 is caused by a coupling error or loss generated during the installation of the primary color light sources, lenses, and beam splitters in a projector.
A red color light emitting diode has a brightness value ranging from 19.0 to 24.5 lm and a wavelength ranging from 615 to 620 nm, a green color light emitting diode has a brightness value ranging from 54.0 to 70.0 lm and a wavelength ranging from 525 to 535 nm, and a blue color light emitting diode has a brightness value ranging from 11.0 to 14.5 lm and a wavelength ranging from 455 to 465 nm.
FIGS. 1 and 2 illustrate a part of CIE 1931 color space and the Planckian locus. In the graphs of FIGS. 1 and 2, the x-axis represents an x chromaticity value and the y-axis represents a y chromaticity value. As shown in Equation (1) below, x and y are expressed by the functions of tristimulus values, X, Y, and Z wherein Y corresponds to the brightness value of a light.
                              x          =                      X                          X              +              Y              +              Z                                      ⁢                                  ⁢                  y          =                      Y                          X              +              Y              +              Z                                                          (        1        )            
FIGS. 1 and 2 plot a Correlated Color Temperature (CCT), CCTMIX, which is a target CCT, D65, 6,500 K, and D93, 9,300K. In FIG. 1, the CCTs are distributed within the range from 6,457 to 18,682 K. In FIG. 2, the CCTs are distributed within the range from 6,669 to 9,504 K.
If an observer can visually recognize the distribution of color temperatures, due to 1) brightness and wavelength variations of primary color light sources or 2) a coupling error or a loss generated in the process of installing primary color light sources, lenses, and beam splitters in the projector, then the color temperatures of the projectors need to be calibrated. However, currently, the color temperatures are not easily calibrated.